There are already games that push those devices to their limits though. I forget, isn't Theatrhythm already barely supported on the 4S? I know on some of the platforms it runs on it turns off a lot of onscreen graphics just to maintain a playable framerate.
And after having used my iPad 3rd gen there's simply no way I can go back to the non-retina screen. It's a night and day difference, especially when you spend as much time reading as I do.
Yup, in iPhone 4 you always get this static ugly red background no matter the song. And despite that it doesn't run very well.